The present invention relates to fluid mixing devices and to demand valves useful with such devices. The invention is particularly useful for blending air and oxygen in order to obtain a mixture having a preselected percentage of oxygen (e.g., for administering to respiratory patients), and is therefore described below with respect to such an application.
At the present time, various types of fluid mixing devices are commercially available for mixing air and oxygen. According to one known method, air and oxygen are inletted via orifices whose cross-sectional area can be preset, as by a control knob, in order to obtain any desired mixture ratio. In another known method, a predetermined quantity of oxygen is introduced into a reservoir, and then ambient air is drawn in to produce the desired mixture. According to a third known method, oxygen is caused to flow at a high rate through a venturi to produce a low pressure which draws air through a nozzle, the air/oxygen ratio being controlled by the opening size of the nozzle.
The above known systems are generally not accurate in delivering a fixed ratio of air/oxygen under a wide variation of flow rates, and/or require extremely accurate hardware to perform the mixing since they depend on size and shape of orifices in a valve or nozzle for this purpose. Some constructions provide accuracy at high flow rates but not at low flow rates, whereas other constructions have high flow resistance at high flow rates, and poor ratio control at low flow rates.
It would therefore be desirable to provide a fluid mixing device of a simple construction which can provide relatively accurate ratio control particularly at a wide range of flow rates.